Hydrochloric acid: An overlooked driver of environmental change

Pollution from fossil fuel burning

Acidification ('acid rain'), including recovery from acidification

Water treatment

Climate change and natural carbon stores

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Acidic pollutants derived primarily from the burning of fossil fuels by power stations and large industrial plants and deposited as “acid rain” have led to the widespread acidification of terrestrial (land) and aquatic (freshwater) ecosystems. Most research in this area has focused on sulphur (S) and nitrogen (N) compounds. Hydrochloric acid (HCl), although also emitted by coal burning, has been largely overlooked as a driver of ecosystem change because most of it was thought to be deposited close to emission sources rather than in remote natural ecosystems.

In the UK, efforts to reduce sulphur emissions, and changes in energy supply away from coal-fired power stations, have had an additional outcome; a 95% reduction in emissions of hydrochloric acid within 20 years (this has been mirrored in other industrialised countries, including the USA and parts of Europe).

Long-term data analysed in this study suggest that the near-disappearance of HCl from deposition could account for 30−40% of chemical recovery from acidification during this time, and that it has had an effect on both near-to-source and remote areas.

In contrast to deposited sulphur and nitrogen compounds that tend to accumulate in wetland soils, HCl tends to pass through these catchments more freely and is therefore likely to have been a more important acidifying pollutant in these systems.

Although the reduction in HCl loadings has enabled natural ecosystems to recover from acidification, there is one arguably less desirable consequence. The shift towards less acidic conditions may be affecting the peatland carbon cycle, contributing to increases in the amount of dissolved organic carbon entering surface waters. This is important for two reasons.

First, dissolved organic carbon (DOC) gives water draining peaty soils its brown colour. When water containing DOC is treated with chlorine in conventional water treatment processes, potentially harmful 'disinfection bi-products' can result. It is important, therefore, that the concentration of DOC is reduced to safe levels before this treatment step. Removing DOC from drinking water is expensive so it is important to understand natural controls on DOC concentration.

Second, peatlands are important stores of carbon. 10-15% of the world's peatlands are found in the UK. Carbon released from peatland soils may eventually re-enter the atmosphere as carbon dioxide or methane, both greenhouse gases. Hence, safeguarding natural carbon stores is an important strategy in our response to climate change.

With many regions of the world increasingly reliant on coal for power generation, hydrochloric acid should be recognized as a potentially significant part of resulting emissions, with distinctive ecosystem impacts.